1. Field of the Invention
The invention relates to a glazing or calendering roll comprising a roll body having a flange journal at each end of the roll body and peripheral passages formed near the roll surface in the roll body for conducting a fluid heat or cold carrier through the roll body.
2. Description of the Prior Art
In heated rolls as used for example in paper machines for glazing the paper web essentially two types of construction have established themselves:
1. tubular rolls with shrunk-in displacement bodies and
2. peripherally drilled rolls.
In the first type a liquid heat carrier flows through an annular gap between the roll tube and the displacement body. In the second type the heat carrier flows through a plurality of bores just beneath the roll surface.
More recent developments in the manufacture of paper machines towards higher surface temperatures and line pressures favour the peripherally drilled roll for various reasons:
At high line pressures large diameters must be chosen to prevent sagging of the rolls and flexural stresses. Larger diameters, which have become increasingly more significant in recent times, lead in tube rolls to an increase of the wall thickness and thus to a further distance of the heat from the heat carrier to the roll surface. Due to the relatively small distance of the peripheral bores from the roll surface which is practically not dependent on the roll diameter, the peripherally drilled roll has the advantage that the path of the heat from the heat carrier through the roll wall to the roll surface is short and thus an effective heat transfer is guaranteed.
With high line pressures larger diameters must be chosen to limit the bowing or deflection of the rolls and the flexural stresses. Excessive deflection or bowing and excessive alternating flexural stresses can only be avoided by increasing the resistance moment of the roll cross-section, which is proportional to the fourth power of the diameter.
This therefore means that larger roll diameters involve a decisive increase in the wall thickness and a pronouced increase in the weight of the roll. This also leads to an increase of the volume of heat carrier fluid which has to be heated on every heating up of the roll or cooled in every cooling operation.
It is therefore obvious, and has already been suggested, to seal the widened inner bore at the ends of the roll body from the heat carrier fluid. However, such a measure also involves disadvantages:
Depending on the connection of the peripheral bores, in driven rolls the fluid carrier must be returned in the roll interior to the inlet journal. For a corresponding tube which is led through the interior of the roll a complicated support structure is required.
The possibility of leakage under temperature-induced thermal stresses in the sealing elements cannot be excluded. Should heat carrier fluid penetrate the roll interior, the result could be a considerable interruption in operation.
In particular during the heating up, the avoidance of contact of the inner roll wall with the hot heat carrier leads to additional tensile stresses at this point, which add to the already present residual inherent stresses originating from the casting of the roll body. The explanation of this is that the outer region of the roll jacket with the peripheral bores heats up fast and expands correspondingly quicker than the inner region.
In rolls of large diameter with correspondingly enlarged inner bores a large amount of heat carrier fluid is necessary in operation. This is not only expensive but as already mentioned the fluid carrier must also be heated up or cooled down every time there is a change of mode of the glazing assembly or calender into which the roll is incorporated. This is not only a waste of energy but additionally causes a lengthening of the heating and cooling operations and accordingly a shortening of the operating time available.
Enlarging the inner bore in rolls of large diameter by drilling also changes the inherent stress distribution of the hard cast rolls in such a manner that the inherent tensile stress increases in the bore wall. Admittedly, this is certainly desirable for high performance heating operations because the superimposed thermal stress counteracts the distribution of the inherent or internal stress and on superimposition of the two stress actions the stress level is diminished. However, in the heating-up phase the temperature increase of the outer roll jacket due to the hot heat carrier in the peripheral bores only follows the temperature increase of the inner roll jacket with a certain delay. This applies in the same manner to the expansion of the roll material at the points. Due to the more rapidly expanding roll material of the shell, the roll bore is subjected to additional tensile stress. In the knowledge of these relationships the admissible heating-up rates are therefore greatly restricted, for example to 0.5.degree. C./min. This means a considerable increase of the waiting times until there is operational readiness; for example, the heating-up time for an operating temperature of 250.degree. C. starting from 20.degree. C. room temperature is almost eight hours.
When a paper web tears and a paper web to be treated is then again led onto the roll, the thermal stresses change in a very short time. With the low admissible temperature change rates it is practically impossible to react to these changes with corresponding heating. The consequences are undesirable quality and thickness fluctuations in the paper web.
Finally, a further disadvantage results from the regulations for approval of various countries. There are countries in which containers with internal pressure fall under certain of the frequently very restrictive regulations governing pressure containers, although as regards the wall thickness they are overdimensioned to such an extent that in other countries they are considered to be machine components. In hard or chilled cast rolls, if the approval regulations are narrowly interpreted, this could lead to considerable problems in obtaining approval because the material "chilled cast iron" is not standardized in the corresponding regulation. Due to the pump pressure for transporting the heat carrier fluid and due to its own vapour pressure even when using heat carrier oils, pressures occur in the interior of the rolls which lie considerably above the ambient pressure.